1. Field of the Invention
The present invention relates to a pulse beam forming method and a pulse beam forming apparatus using a charged particle beam such as an electron beam and an ion beam.
2. Description of the Related Art
A pulse beam with a pulse width of picosecond order is mainly used in an EB (electron beam) tester. A beam deflecting method is employed as a pulse beam forming method, in which an electron beam is deflected by using a transverse electric field and is cut by means of an aperture. This method has advantages such as the apparatus employing the method can be easily mounted on a scanning electron microscope (SEM) and that a pulse width of nanosecond to picosecond order can be readily obtained.
FIG. 3 shows a known pulse beam forming apparatus for an EB tester and a ray diagram thereof. As shown in FIG. 3, an electron gun comprises a cathode 101, a Wehnelt cathode 102, and an anode 103, which emits an electron beam 110 having a substantially circulars cross section in an accelerated state. The electron beam 110 radially emitted is converged through a condenser lens 106 and an objective lens 107 to form an image of the cathode on a sample surface 109. There is an objective aperture (chopping aperture) 108, having a circular opening and disposed between the condenser lens 106 and the objective lens 107, to define an opening angle of the beam. A deflector 104 is located between the condenser lens 106 and the electron gun. When a sinusoidal wave or pulse is input from a power source 105 into the deflector 104, the electron beam 110 is deflected by the deflector 104. The objective aperture 108 periodically cuts the electron beam 110 to form a pulse beam.
FIG. 4 schematically shows a relation among the chopping aperture, the deflector, and the electron beam. An electron beam 210 is deflected by a parallel-plate type deflector 201, and is cut by a chopping aperture 202 (a conjugate image of objective aperture). A pulse width .tau. of the thus-obtained pulse beam may be expressed in a simple form by the following equation: EQU .tau.=2.times.S.times.V.times.(d+D)/k.times.h.times.L (1),
where D is the opening inner diameter of the chopping aperture 202 and d is the beam diameter on the aperture 202.
In the equation (1), S is the distance between electrodes of the deflector, h is the length of the electrodes of the deflector, V is the acceleration voltage of the electron beam, L is the distance from the deflector to the chopping aperture, and k is the velocity of change of the deflection voltage. As seen from the equation (1), the pulse width .tau. is proportional to the sum (d+D), which is a sum of the opening inner diameter D of the chopping aperture and the beam diameter d on the aperture.
As seen from the equation (1), it is necessary to make the sum (d+D) smaller in order to form a pulse beam of shorter pulse width. The opening inner diameter D of the aperture determines the performance of the objective lens 103 (FIG. 3) and is determined by electron optic conditions. On the other hand, the beam diameter d on the aperture cannot be made smaller than the aperture opening. If the sum (d+D) should be made smaller, the beam current of the electron beam passing through the aperture 202 would decrease. Particularly, when an electric field emission type electron gun is employed as the electron gun, the reduction rate of the optical system will decrease. Therefore, the opening inner diameter D of the chopping aperture must be kept relatively large in order to obtain a certain beam current.
As described, the sum (d+D), which is a sum of the opening inner diameter D of the aperture 202 and the beam diameter d on the aperture 202, must be made smaller in order to obtain a pulse beam of shorter pulse width. However, if the sum (d+D) is to be made smaller, the beam current of the electron beam decreases so as to lower the transmittance of the electron beam.